In a vehicle, as shown in FIG. 5, a float valve 100 is provided to the upper part of a fuel tank 200 for guiding, to a canister 300, fuel gas to be generated in the fuel tank. The float valve 100 and the canister 300 are connected to each other by a connecting pipe 400, and the fuel gas of the fuel tank 200 is guided to the canister 300. This float valve 100 eases the rapid increase of internal pressure of the fuel tank 200, and prevents a back flow of the fuel from a fuel filler opening 250.
Here, the float valve of a conventional type is shown in FIG. 6.
The conventional float valve 100 is provided with a case 101 having a space formed inside, a valve port 102 that opens toward the upper surface of the case 101, a float 105 to be housed in a space 103 of the case 101 to freely move therein, a valve body portion 106 that is formed to protrude from the upper surface of the float 105 so as to close the valve port 102, a spring 107 that is provided inside of the float 105 so as to bias the float 105 upward, and a vent 108 that is formed on the side surface of the case.
To the upper part of the case 101, a connecting pipe that is not shown is connected for guiding the fuel gas to a canister that is not shown. Herein, although the spring 107 always biases upward the float 105 with a load smaller than the self weight of the float 105, it does not push up the upright float 105 unless the buoyant force acts.
In the below, the movement of such a float valve 100 is described.
As shown in FIG. 6, when a fuel level L is located at a lower position, the float 105 moves down due to its self weight, and is located at the lower position in the case 101. Therefore, the valve body portion 106 is sufficiently away from the valve port 102.
In this state, the valve body portion 106 does not interrupt the connection between the space 103 of the case 101 and the connecting pipe 400 to the canister 300, and the fuel gas generated inside of the fuel tank 200 is guided from the vent 108 to the canister 300 via the space 103 and the connecting pipe 400. With such a structure, the internal pressure of the tank can be appropriately maintained, and the fuel supply can be performed in a safe and stable manner.
Next, when the liquid surface of the fuel exceeds a predetermined level in the fuel tank 200 due to a fuel increase in the fuel tank 200 as a result of fuel supply, too much banking of a vehicle, or overturning thereof, the float 105 moves up by receiving the buoyant force, and the valve body portion 106 closely attaches to the valve port 102 so that it becomes in the valve-closed state. Thereby, the fuel leakage is prevented (refer to Patent Document 1, for example).
However, such a conventional float valve as above is required to have the valve port 102 of a large diameter for the purpose of exhausting the fuel gas with efficiency to reduce the pressure loss at the time of fuel supply, for example. As a result, the valve body portion 106 is also increased in diameter.
The issue here is that when the liquid surface inside of the fuel tank 200 moves down, the float 105 moves down due to its self weight (the load derived by subtracting the spring's biasing force from the self weight of the float itself) . When the pressure in the fuel tank 200 differs a lot from the pressure in the connecting pipe 400 on the side of the canister 300, and when the pressure in the fuel tank 200 is higher, the upward force acting on the valve body portion 106 exceeds the self weight of the float 105, causing such a trouble that the valve is prevented from opening and the valve body portion 106 is attached to the valve port 102. Such an attachment force of the valve body portion 106 with respect to the valve port 102 is increased proportionate to the pressure reception area of the valve body portion 106.
Therefore, when the liquid surface inside of the fuel tank 200 moves down, the float 105 is supposed to go down due to its self weight and open the valve body portion 106. However, when the valve port 102 and the valve body portion 106 are both increased in diameter as described above, it may possibly cause a problem that the attachment force of the valve body portion 106 is increased with respect to the valve port 102, and even if the liquid surface is lowered, the valve body portion 106 is not released from the valve port 102 and thus is not opened.
Here, in order to solve such an attachment problem of the valve body portion 106, proposed is a float valve with which the open valve characteristics can be stably maintained even if the pressure reception area of the valve body portion 106 is increased by preventing attachment through increase of the weight of the float 105 (as an example, refer to Patent Document 2).
Patent Document 1: JP-A-2002-115613
Patent Document 2: JP-A-10-89182